Complex determination of male sterility in Thymus vulgaris L.: genetic and molecular analysis.

Nucleocytoplasmic determination of male sterility in Thymus vulgaris L. has been assumed in all papers attempting to explain the remarkably high frequencies of male steriles found in natural populations of this species. This paper provides strong evidence that both nuclear and cytoplasmic genes are involved in the determination of male sterility of this species, giving a complex inheritance. Interpopulation and intrapopulation crosses have shown that the ratio of females versus hermaphrodites among offsprings varied widely from 1∶0 to 1∶1. Furthermore, interpopulation crosses consistently yielded a higher frequency of females than intrapopulation crosses. Nucleocytoplasmic inheritance was demonstrated by an absence of male fertiles in backcrosses and asymmetrical segregation in reciprocal crosses. Molecular analysis of the mitochondrial DNA of some of the parents used in crosses suggested the involvement of different cytoplasms in the inheritance of male steriliy.